Question

The electrophile involved in the sulphonation of benzene is

• A. SO\(_3^+\)
• B. SO\(_3^{2-}\)
• C. H\(_3\)O\(^+\)
• D. SO\(_3\)

Hint:

SO\(_3\) is a strong electrophile due to electron-deficient sulfur.

Answer 1

The Correct Option is D

To solve the problem of identifying the electrophile involved in the sulphonation of benzene, we need to understand the mechanism of this reaction in organic chemistry.

Benzene sulphonation is a classic electrophilic aromatic substitution reaction where benzene reacts with concentrated sulfuric acid (H\(_2\)SO\(_4\)) to form benzenesulfonic acid. The key electrophile in this process needs to be identified.

Sulphonation involves the reaction of benzene with sulfur trioxide (SO\(_3\)), which is often generated in situ from sulfuric acid. This is represented by:

1. \(2 \text{H}_2\text{SO}_4 \rightarrow \text{H}_3\text{O}^+ + \text{HSO}_4^- + \text{SO}_3\)

SO\(_3\) is highly electrophilic because it lacks a complete octet around sulfur, making it eager to accept electrons. Thus, it acts as the electrophile in the reaction with benzene.

The mechanism can be summarized as follows:

• The pi electrons in the benzene ring attack the electron-deficient sulfur in SO\(_3\), forming a sigma complex (arenium ion).
• This intermediate rapidly loses a proton to regenerate the aromaticity of the benzene ring, resulting in the formation of benzenesulfonic acid.

Therefore, among the given options, \(\text{SO}_3\) is the correct electrophile involved in the sulphonation of benzene.

To summarize, the correct answer is \(\text{SO}_3\) because it is the species that acts as the electrophile in the sulphonation reaction, attacking the electron-rich benzene ring.